In general, when drilling hydrocarbon wells, a drill bit is disposed at the end of a drill string, and typically, the drill string is rotated from the surface utilizing either a top drive unit or a rotary table set in the drilling rig floor. As drilling progresses, increasingly smaller diameter tubulars comprising casing and/or liner strings may be installed end-to-end to line the borehole wall. As the well is drilled deeper, each string is run through and secured to the lower end of the previous string to line the borehole wall. Finally, the string is cemented into place by flowing cement down the flowbore of the string and up the annulus formed by the string and the borehole wall.
To perform the cementing operation, a cementing manifold is usually disposed between the top drive unit or rotary table and a work string extending into the well. Due to its position, the cementing manifold must suspend the weight of the work string and the casing string, contain pressure, transmit torque, and allow unimpeded rotation of the work string. The cementing manifold is designed to allow fluids, such as drilling mud or cement, to flow therethrough while simultaneously enclosing and protecting from flow one or more darts that are released on demand and in sequence to perform various operations downhole, including wiping pipe surfaces, separating fluids, and actuating downhole tools. Thus, as fluid flows through the cementing manifold, the darts are isolated from the fluid flow until they are ready for release.
Within the borehole, the work string, with one or more cementing plugs disposed at a lower end thereof, extends into and connects to a casing running tool that suspends the casing string to be cemented. Thus, the work string is positioned upstream of the casing string. The work string runs the casing string into the borehole to the desired depth, and the casing string fills with drilling fluid or other fluid in the well as it is being run in. When the casing string is positioned at the desired depth, cement is pumped downhole through the work string. As the cement is pumped, a dart or other device is released from the cementing manifold and propelled down the work string ahead of the batch of cement. The dart lands in a seat in one of the cementing plugs at the lower end of the work string, and the pressure behind the dart causes the cementing plug to be released as the cement pushes the plug down. Thus, the cementing plug is released by the dart ahead of the cement batch. This cementing plug maintains a separation between the cement slurry and the drilling fluid, and thereby reduces contamination of the cement slurry as it flows into the casing string. The cementing plug that precedes the cement slurry and separates it from the drilling fluid is referred to herein as the “bottom cementing plug.” This bottom cementing plug also sealingly engages the inner surface of the casing string to wipe the drilling fluid from the walls of the casing string ahead of the cement slurry. The bottom cementing plug then lands on a float collar or float shoe attached within the bottom end of the casing string.
When the bottom cementing plug lands on the float collar or float shoe attached to the bottom of the casing string, a bypass mechanism in the bottom cementing plug is actuated to allow the cement slurry to proceed through the bottom cementing plug, through the float collar or float shoe and upwardly into the well bore annulus between the casing string and the borehole wall. When the required quantity of cement slurry has been pumped through the work string, a second dart or other device is launched from the cementing manifold to follow the cement batch. This dart is pushed along by a displacement fluid and wipes cement from the walls of the work string, then lands in a releasing sleeve of a second cementing plug at the lower end of the work string. The second cementing plug, referred to herein as the “top cementing plug”, is thereby released from the work string to separate the cement slurry from additional drilling fluid or other fluid used to displace the cement slurry through the casing string. The design of the top cementing plug is such that when it lands on the bottom cementing plug at the lower end of the casing string, it shuts off fluid flow through both the top and bottom cementing plugs, which prevents the displacement fluid from entering the well bore annulus.
The traditional cementing method described above involves a cementing manifold that comprises an integral part of the work string, thus requiring field personnel to work in close proximity to the work string to manually release darts during cementing operations. When operating from a drilling platform, for example, field personnel may be wenched into a harness and suspended from a derrick within reach of the cementing manifold so that such personnel may manually manipulate valves to release darts into the work string at desired times. This manual method of releasing darts creates the risk of injury to field personnel, especially when operating from an offshore floating platform, for example, where wind and waves create additional hazards to personnel suspended in a harness. Therefore, such manual methods of releasing darts from conventional cementing manifolds are not permitted in some countries, such as Norway, for example. Thus, an alternative method of releasing darts from the cementing manifold was developed wherein the cementing manifold valves are remotely actuated rather than manually actuated. Although such remote actuation methods address the concern about field personnel working in close proximity to the work string, the cementing manifold must still be capable of suspending the weight of the work string and casing string, containing pressure, transmitting torque, and rotating the work string. In addition, remote actuation of the cementing manifold valves to release darts adds complexity to the system, and therefore more cost and less reliability as compared to the traditional manual method using field personnel to manipulate the valves.
Thus, a need exists for apparatus and methods to remotely release actuating, wiping, and/or separating devices, such as mechanical plugs, into the work string during cementing operations, while reducing design complexity of the cementing manifold, reducing manufacturing costs, and increasing operational reliability.